A large number of electronic units require that the alternating current line voltage be converted to a direct current. For that purpose, power supplies in the form of switched converters and inverters have been proposed in the prior art. Such circuits usually involve a large storage capacitor connected across a rectifier bridge output, and this causes the input line current to be highly non-sinusoidal. Accordingly, poor effective power factors are encountered which result in higher input current for a given output of the power supplies.
Specifically, a conventional switched power supply operates as a capacitive load for a rectified line voltage. Such a load draws relatively high currents near the peaks of the alternating current cycle, and substantially zero current for the remainder of the cycle. This results in poor power factor which manifests itself in a larger RMS current for a given input voltage and power load than if the load were purely resistive. A typical prior art switched power supply, for example, draws about 1.5 times the RMS current for a given power output, as compared with a purely resistive load. Consequently, for a given line current the usual prior art switched power supply is capable of delivering only about two-thirds the power which could be delivered if the line current were sinusoidal, which is the case when the load is purely resistive.
A switched mode unity power factor power supply is described in U.S. Pat. No. 4,677,366 which includes a boost converter between a diode rectifier bridge and the storage capacitor included in the power supply. The boost converter is incorporated into the power supply because it draws a relatively smooth current from the power line, and also because it permits the voltage on the storage capacitor to be higher than the voltage produced by the diode rectifier bridge which provides for more efficient energy storage. The boost converter draws a sinusoidal current by operating as a current regulator with a current reference control signal set to track the line voltage waveform to control the voltage across the storage capacitor. The voltage across the storage capacitor is produced by controlling the magnitude of the current reference control signal, but not its wave shape, by a regulating signal. By regulating the voltage across the storage capacitor at a high level, excellent power hold-up time can be achieved when the line power is removed regardless of the input line voltage, with much less storage capacitance being required as compared with the storage capacitance requirements of the previous types of switched power supplies.
The present invention is concerned with a simplified technique and circuit for reducing line conducted electromagnetic interference (EMI) in a pulse-width modulated power factor corrector (PFC) circuit without any need to increase the size of the EMI line filter which is usually incorporated into PFC circuits.
Systems for reducing line conducted electromagnetic interference (EMI) in switched mode power supplies are known to the art. One such system is described, for example, in U.S. Pat. No. 4,712,169 which issued Dec. 8, 1987 in the name of Manfred Albach.
The Albach system and the system of the present invention are similar to one another in that each utilizes the alternating current line voltage to vary the frequency of the switching pulses of a switched mode power supply in order to spread the EMI over a wide spectrum. This procedure is carried out so that the detected EMI may be reduced without having to increase the size of the EMI filter.
However, in the Albach system, the frequency is varied under the control of the alternating current line voltage in a manner such that maximum frequency is achieved at minimum voltage of the AC line, and minimum frequency is achieved at the maximum peaks of the AC line voltage.
Because the frequency in the Albach circuit is a minimum at the voltage peaks, Albach is required to provide complex circuitry including a comparator and a voltage controlled oscillator to achieve the desired reduction in EMI.
The system of the present invention, on the other hand, includes a simple circuit for injecting a current proportional to the instantaneous line voltage into the timing capacitor of a pulse width modulator which is included in the circuit. This connection causes the charging cycle to speed up at the peaks of the line voltage and hence increases the switching frequency at the voltage peaks. When the alternating current line voltage is not at a voltage peak, the speed-up and hence the frequency is less. Thus, the system of the present invention, contrary to the teaching of Albach, provides for the frequency to increase at the peaks of the alternating current line voltage, and to decrease as each cycle of the line voltage decreases.
The system of the present invention, accordingly, preserves the simple control loop and continuous PFC choke current of the pulse width modulator, while giving the noise spreading advantage of varying PFC frequency. Also, the frequency variation in the system of the invention can be tailored to operate at lower frequencies for 115 VAC input where the line current and losses are highest (lower frequency equals lower switch loss); and at higher frequencies at the 230 VAC input where EMI restrictions are tightest but line currents are lower. Higher frequencies require lower inductance and capacitance to perform the appropriate filtering function.
While the frequency variation technique referred to above will be described in conjunction with off-line power factor correctors, it also can be used for downstream DC-to-DC converters that are pulse width modulated to reduce their contribution to the line conducted and/or radiated EMI.